This invention relates to contact copying of reflection or volume holograms.
In the mass production of reflection or volume holograms it is usual to prepare a transmission master hologram of an object and from this to prepare a copy reflection master hologram and from the reflection master hologram to prepare by contact copying a large number of reflection holograms. Such contact copying is usually accomplished by placing in register the reflection master hologram and a sheet of transparent photosensitive holographic material. An overall exposure to laser light is then used to form a holographic copy image in the photosensitive material. However, unless a very high power laser is used to effect such an overall exposure, a comparatively long exposure time is required. During this time often the holographic material and the hologram alter their relative position and a hologram with unclear fringes is obtained. To overcome the need to use a long exposure time it has been proposed to employ a scanning method wherein a dot of high intensity laser light raster is employed or a narrow line of lower intensity laser light is linearly scanned so as to sweep out a rectangular area of exposure of the holographic material through the hologram. A scanning method of this type reduces very greatly the exposure time required but it is cumbersome and expensive to set up such a scanning system. In addition, the use of a reflection hologram as the master from which the copies are made has four serious disadvantages:
(i) the master hologram reflects a constant fraction of the light incident as it is in a given environment. Thus there can be no control over the reference to object beam intensity ratio hereinafter referred to as the beam ratio, a parameter which significantly affects the brightness of the final image.
(ii) in order to achieve sufficient reflectivity for good copying the wavelength of maximum reflectivity of the reflection master must closely match the wavelength of the copying laser. This can often require control of complicated swelling processes in order to compensate for shrinkages in the reflection master material during processing or for differences in wavelength between the laser used for recording the transmission master and the laser used for copying.
(iii) as the relative humidity changes a standard reflection master changes its reconstruction wavelength. The reflectivity at the laser wavelength changes, and consequently the brightness of the copy changes. To overcome this instability may require considerable expenditure on air conditioning equipment.
(iv) shifts in the replay wavelength during the copying process can adversely affect the reference angle required for replay of the copy and the viewing angle of the copy.
There is limited scope to correct these defects. A scanning system to provide holograms is described for example in British patent application Ser. No. 2159979 but this does not appear to be a copy system. A scanning system to provide a transmission hologram from a transmission master by a scanning system is described in Japanese patent application Ser. No. 56-215101 but in this case the transmission copy is a surface relief type hologram on a resin plate and not a light-transmissive holographic copy.
On the other hand the use of a transmission hologram as the contact master has no such disadvantages. When using a transmission hologram as the master from which to produce reflection copies the beam ratio can be controlled and compensation can be made for material shrinkages or differences in laser wavelength by adjustment of the angles of incidence and intensity of the beam used to replay the transmission master and/or the copying reference beam.
Various methods for preparing a reflection copy from a transmission master have been proposed but in these methods a single overall exposure is postulated and not a scanning exposure. One such prior art proposal is described in the Annales de Radioelectricite No. 88 Apr. 1967 on pages 105-108. The details given in this short article are limited but it is proposed that a split beam from a laser strikes two reflecting surfaces to expose the holographic material via the transmission master.
Another such prior art proposal is described in U.S. Pat. No. 3647289 wherein an improved copy reflection hologram is obtained from a master transmission hologram using a single overall exposure by placing between the holographic material and the transmission master means to cut out the zero order light (or the undiffracted beam). But the system described in U.S. Pat. No. 3647289 is not a scanning system.